Allow a specified output name in bgzip

[whitwham]

Choose a specific output name for a bgzipped file. e.g.

bgzip file.txt -o myfile.txt.gz

This will overwrite files without warning but will not change the existing source files and it will not conserve file modification dates.

Fixes #1726.

[jkbonfield]

More complex than I expected indeed.

It passed all the things I could think of bar one. "-" as a synonym for stdin/stdout works for -o when compressing but not when decompressing. bgzip -d -o - -k foo.gz writes to a file named "-".

[whitwham]

There is some special code to handle stdout already present. I've changed it so "-o -" is treated as -c instead.